Bleching of cellulose fibers with an aqueous solution of a water-soluble hydroperoxypentacyanocobaltate salt

ABSTRACT

CELLULOSE FIBERS ARE BLEACHED BY CONTACT WITH AN AQUEOUS SOLUTION OF A WATER-SOLUBLE HYDROPEROXYPENTACYANOCOBALTRATE SALT. THE SPENT COBALTATE SALT IS REGENERATED AND RECYCLED.

United States Patent Office 3,733,247 Patented May 15,, 1973 3,733,247BLEACHING F CELLULOSE FIBERS WITH AN AQUEOUS SOLUTION OF A WATER-SOLU-BLE HYDROPEROXYPENTACYANOCOBALTATE SALT John Allan Fetchin, Stamford,Conn., assignor to American Cyanamid Company, Stamford, Conn. NoDrawing. Filed Nov. 18, 1971, Ser. No. 200,163 Int. Cl. D21c 9/16 US.Cl. 16278 10 Claims ABSTRACT OF THE DISCLOSURE Cellulose fibers arebleached by contact with an aqueous solution of a water-solublehydroperoxypentacyanocobaltate salt. The spent cobaltate salt isregenerated and recycled.

The present invention relates to the bleaching of cellulose fibers byuse of a bleaching agent. The invention includes the process wherein thebleaching agent is regenerated and if desired recycled.

Paper (including paperboard, cardboard and similar products) is commonlymade by slurrying wood pulpin water, flowing the resulting fibroussuspension upon a screen to form a wet web, removing the web from thescreen and drying the web. It is particularly desired that the finalproduct should have a brilliant white color and to this end it is commonpractice to subject the pulp to a bleaching step. Moreover, paper isincreasingly manufactured from recycled paper, the fibers of which maycarry a dye and this presents a bleaching problem. 'In the manufactureof cotton textiles, it is frequently desired that the product have amaximum attainable whiteness.

The bleaching of fibers intended for use in the manufacture of paper andtextiles is commonly performed by the use of strong oxidizing agents,for example, sodium hypochlorite, hypochlorous acid, tertiary butylhydroperoxide, sodium peroxide, sodium peroxyacetate, and benzoylperoxide. It is a disadvantage of these processes that it is notpractical to recycle or otherwise reuse spent oxidizing solution, andaccordingly the efiiuent liquor from the oxidizing step is commonlydiscarded. Strong oxidizing agents are detrimental to life, and theintroduction of these agents into lakes and streams causes an ecologicalproblem. Moreover, the discharge of such liquors involves a waste of theunconsumed peroxide which they generally contain.

The discoveries have now been made that the watersolublehydroperoxypentacobaltate salts are highly effective bleaching agentsfor cellulose, capable of causing a substantial increase in thebrightness even of cellulose fibers which have been bleached byconventional processes; that they perform this bleaching action rapidlyand safely (if desired at room temperature); that the cobaltate inreduced state in the liquor coming from the process can be oxidized toperoxy form by a simple and industrially practical process, therebypermitting the liquor to be recycled for the treatment of subsequentbatches of fiber, and that this oxidation can be performed in apparatuswhich does not require pressure.

Broadly, then the invention is the bleaching of either bleached orunbleached cellulose fibers by contacting the fibers with an aqueoussolution of a water-soluble hydroperoxypentacyanocobaltate salt. In alimited sense the invention is the bleaching step followed byregeneration and, if desired, the recycling of the bleaching solution.

The invention is performed cyclically by contacting the fibers to bebleached with an aqueous alkaline solution of ahydroperoxypentacyanocobaltate salt thereby bleaching at least part ofthe fibers and simultaneously reducing at least a part of said cobaltateto hydroxypentacyanoco- 'baltate salt, separating the solution from thefibers, converting at least a part of the latter cobaltate back to ahydroperoXy-pentacyanocobaltate salt, and recycling the solution.

It has further been found that regeneration of the spenthydroperoxypentacyanocobaltate from the bleaching step can be performedby reducing the spent cobaltate to a pentacyanocobaltate salt, stillfurther reducing said pentacyanocobaltate salt to ahydridopentacyanocobaltate salt, and oxidizing saidhydridopentacyanocobaltate. The product is the desiredhydroperoxypentacyanocobaltate salt, which can be recycled.

It has still further been found that conversion of said spenthydroperoxypentacyanocobaltate salt to a hydroperoxypentacyanocobaltatesalt is performed by reacting said hydroxypentacyanocobaltate salt withhydrogen and aluminum, thereby forming a pentacyanocobaltate salt, andreacting said pentacyanocobaltate with molecular hydrogen therebyforming a hydridopentacyanocobaltate salt, and reacting said latter saltwith molecular oxygen.

The aforesaid reactions can be performed with gas pressures betweenatmospheric pressure and 100 atmospheres and at temperatures from 0 C.to room temperature or higher.

The theoretical reactions involved in the regeneration of the reducedcobaltate to the active peroxy form are as follows (assuming potassiumto be the cation), together with the names of the anions formed. In theequations the term PCC designates pentacyanocobaltate.

K3HOOC0(CN)5 color K3HOCO(CN)5 bleached Hydride P CO Hydroperoxy P C CThe above reactions are also applicable to any watersoluble cobaltatesalt, especially the alkali and alkaline earth metals salts, forexample, the sodium, tetramethylammonium, calcium and magnesium salts.The sodium and potassium salts are preferred because they areinexpensive and provide good results. Any of the previously mentionedcations can be employed in place of the potassium cations shown in theequations above, with substantially the same results.

The bleaching step is carried out simply by contacting pulp with anaqueous solution of a hydroxypentacyanocobaltate salt. Even a smallamount of the cobaltate elfects some bleaching, from which it appearsthat there is no amount of cobalate however small or duration of contacthowever short which will not effect at least some brightening. From thegeneric point of view therefore the invention does not depend uponamounts, times, temperatures, etc. However, in practice between about0.001% and 20% of the cobaltate (based on the dry weight of the fibers)is needed to make the process economic, and best bleaching occurs whenthe amount of the cobaltate is in the range of 0.01% and 10% based onthe fibers. With regard to the concentration of the cobaltate in thebleaching solution, the data indicate that bleaching proceeds betterwhen the concentration of the cobaltate in the solution is low ratherthan high.

The bleaching step can be performed at room temperature. However, morerapid and extensive bleaching occurs when the step is performed atelevated temperature, e.g., in the range of 40 C.- C.

The bleaching process can be performed over the pH range of 3 to 13,which includes the papermaking range.

The alkali metal salts of hydroperoxypentacyanocobaltate are basic, andtherefore the solutions used in the process of the present inventioninherently have an alkaline pH unless otherwise desired. The solutionacts as a bleach even when it has a content of free alkali. For thispurpose any alkali metal or soluble alkaline earth metal hydroxide canbe added, as well as soluble quaternary ammonium hydroxides.

Upon completion of the bleaching step the liquor is separated from thefibers in any convenient manner, for example, by centrifugation,filtration, compression, etc.

The resulting fibers may be processed directly into paper or textiles.However, better brightness is generally obtained when the fibers arewashed after the bleaching step and before it is introduced into thepapermaking process. Evidently, the step of washing removesdecomposition products from the pulps as well as unreacted and spentcobalt complexes which to some extent remain lightly 2 afiixed to thecellulose. Best washing is achieved with water.

The conversion of the reduced peroxy cobaltate in the liquor from thebleaching step to peroxy state can be performed according to the processof the present inven- 2 tion in several steps by several methods. In thefollowing description the term salt includes mixtures of anywater-soluble salts, and references to potassium are for exemplarypurposes only.

In the first step the spent bleaching component is reduced to apentacyanocobaltate salt (see Equation 2 above). Suitable reducingagents are the combination of molecular hydrogen with a water-solublehydridopentacyanocobaltate salts or molecular hydrogen with palladium oncharcoal, aluminum powder, or sodium borohydride. Catalysts such as(e.g., Raney nickel) are used for hydrogenation reactions are alsosuitable. If desired, before or during this reaction, the solutions maybe photolyzed to hydrolyze any K Co(CN) or similar salts to or KHOCo(CN) or similar salts, which are more reactive. It is also possibleto reduce the spent cobaltate component electrolytically.

In the second step (cf. Equation 3 above) the solution is subjected tothe action of hydrogen alone. Pressures from atmospheric to at least 100atmospheres are suitable. This step is automatically performed by theaction of molecular hydrogen. In this step the pentacyanocobaltate saltis converted to the corresponding hydridopena tacyanocobaltate salts.

In the third step (Equation 4 above) the hydridopentacyanocobaltate saltis reacted with molecular oxygen at pressures from atmospheric pressureto 100 atmospheres or more and at temperatures from 0 C. to roomtempera- 0o ture. In this step the hydropentacyanocobaltate salt isoxidized to an appropriate hydroperoxypentacyanocobaltate.

The process is applicable in general to the conversion of a hydrideligand to a hydroperoxy ligand coordinated to cobalt.

The solutions resulting from the foregoing treatment methods can berecirculated directly to the bleaching step without interveningtreatment.

The invention is further described by the examples 6 which follow. Theseexamples illustrate the invention and are not to be construed aslimitations thereon.

EXAMPLE 1 The following illustrates the effectiveness of the process ofthe present invention in brightening a pulp which has previously beenbleached.

3.0 g. (dry basis) of a kraft hardwood pulp which had previously beenbleached to a brightness of 48.4% (reflectance of pulp as the percent ofthe reflectance of BaCO 457 mg, is slurried in 97 cc. of water at 60 C.containing 3.3 g. of tripotassium hydroperoxypentacyanocobaltate [forpreparation see J. Chem. Soc. 397 (1968); J. Catalysis 3, 123 (1964);and Aust. J. Chem. 16, 954 (1963)]. The slurry is gently stirred for afew moments to distribute the cobaltate solution uniformly throughoutthe pulp, and the slurry is allowed to stand for 60 minutes at 60 C. Thefibers are then recovered by filtration, washed three times with waterand once 10 with very dilute (0.1%) sulfuric acid, and dried. They havea brightness of 62.2% (an increase of 13.3 percentage points).

EXAMPLE 2 The following illustrates the bleaching action of potassiumhydr0peroxypentacyanocobaltate at various dilutions on kraft hardwoodpulp.

An aqueous solution is used containing 3% by weight of potassiumhydroperoxypentacyanocobaltate. The pH of the solution is alkaline.Aliquots are taken from a hot laboratory stock of bleached hardwoodfibers having a brightness of 48.4% and a consistency of 3% heated to 75C. To these are added a sufficient amount of an aqueous solution oftripotassium hydroperoxypentacyanocobaltate to provide the amount of thecobaltate shown in the table below. The aliquots are agitated gently todistribute the cobaltate therein. The aliquots are maintained at 75 C.for 90 minutes, after which the fibers are recovered by filtration,washed and dried, and their brightness redetermined. The filtrate isanalyzed to determine the percent of hydroperoxy s-ubstituents whichwere reduced.

Results are as follows:

Based on dry Weight of pulp.

b Percent of cobaltate consumed (i.e., reduecd).

c Reflectance against barium carbonate standard.

d Grains cobaltate required to increase brightness of 100 g. of pulp by1.0 percentage point.

EXAMPLE 3 The following illustrates the treatment of a solution oftripotassium hydroperoxypentacyanocobaltate containing tripotassiumhydropentacyanocobaltate (the spent liquor from the bleaching ofcellulose pulp) to oxidize the hydroxypentacyanocobaltate contentthereof to hydroperoxycobaltate. The solution contains 1.6% oftripotassium hydroxypentacyanocobaltate.

To the solution is added 0.054 g. of powdered aluminum as reductant andthrough the solution with stirring is bubbled hydrogen (from a porousdisc sparger) for 105 minutes. Much of the hydroxypentacyanocobaltate isthereby converted to tripotassium hydridopentacyanocobaltate.

The unreacted aluminum is removed by filtration. The solution is thensubjected to a pressure of 85 1b./in. of O in a rocking autoclave atroom temperature, converting a part of the latter cobaltate to thedesired hydroperoxide.

Analysis shows that this treatment converts 71% of the starting reducedcobaltate to the desired peroxy a cobaltate.

This solution is used for the brightening of pulp by the process ofclaim 1. Substantially the same brightening is achieved.

EXAMPLE 4 The following illustrates a method for the regeneration of thehydroperoxycobaltate by a method wherein an alkali metalhydridopentacyanocobaltate and molecular hydrogen are used together toreduce the hydropentacyanocobaltate.

A pulp is bleached with a solution of potassiumhydroperoxypentacyanocobaltate as shown in Example 1. A part of theresulting aqueous medium is preserved, and the remainder is treated witha reducing agent to produce a solution of tripotassiumpentacyanocobaltate which in turn is reduced to tripotassiumhydridopentacyanocobaltate, also as shown in Example 1 and illustratedby Equations 1-3.

Equimolar quantities of the tripotassium hydridopentacyanocobaltate andthe tripotassium pentacyanocobaltate solutions are mixed under a blanketof nitrogen and the mixture sparged with hydrogen at room temperaturefor an hour. The following reactions occur:

The resulting solution containing the K HCo(CN) produced by Equation 3is reacted with molecular oxygen by the method of Example 3. Titrationof the reulting solution shows that 34.4% of the original K3H0C0(CN)5 isconverted to K HOCo (CN) by this method.

I claim:

1. Process for the bleaching of cellulose fibers which comprisescontacting said fibers with an aqueous solution of a water-solublehydroperoxypentacyanocobaltate salt thereby bleaching said fibers.

2. A process according to claim 1 wherein the cobaltate salt istrisodium hydroperoxypentacyanocobaltate.

3. A process according to claim 1 wherein the pH of the solution iswithin the range of 9.5 to 12.5.

4. A process according to claim 1 wherein the fibers are unbleachedkraft hardwood fibers.

5. A process according to claim 1 wherein the fibers are kraft hardwoodfibers which have previously been bleached.

6. A cyclic process for the bleaching of cellulose fibers whichcomprises contacting oil-color fibers with an aqueous alkaline solutionof a hydroperoxypentacyanocobaltate salt thereby bleaching at least apart of said fibers and simultaneously converting at least a part ofsaid cobaltate to a hydroxypentacyanocobaltate salt, separating saidsolution from said fibers, converting at least a part of said lattercobaltate back to a hydroperoxypentacyanocobaltate salt, and recyclingsaid solution.

7. A process according to claim 6 wherein conversion of saidhydroxypentacyanocobaltate salt to a hydroxyperoxypentacyanocobaltatesalt is performed by further reducing said hydroxypentacyanocobaltate toa pentacyanocobaltate salt, still further reducing saidpentacyanocobaltate salt to a hydridopentacyanobaltate salt, andoxidizing said hydridopentacyanocobaltate salt.

8. A process according to claim 6 wherein conversion of saidhydroxypentacyanocobaltate salt to a hydroperoxypentacyanocobaltate saltis performed by reacting said hydroxypentacyanocobaltate salt withhydrogen and aluminum, thereby forming a pentacyanocobaltate salt,reacting said pentacyanocobaltate salt with hydrogen thereby forrning ahydridopentacyanocobaltate salt, and reacting saidhydridopentacyanocobaltate salt with molecular oxygen.

9. A process according to claim 8 wherein the reactions of said hydrogenare performed between atmospheric pressure and atmospheres.

10. A process according to claim 8 wherein the reaction of said oxygenis performed between atmospheric pressure and 100 atmospheres.

References Cited UNITED STATES PATENTS 2,394,989 2/1946 Dreyfus 162-78 X3,156,654 11/1964 Konecny et al. 252-186 X ROBERT L. LINDSAY, JR.,Primary Examiner A. L. CORBIN, Assistant Examiner US. Cl. X.R.

